1. Field of the Invention
The present invention relates generally to apparatus and methods for treating and removing occluding material from a body lumen, such as blood vessels. In particular, the present invention relates to apparatus and methods for guided atherectomy.
2. Description of the Prior Art
A number of methods and devices are currently available for removing stenotic or occluding material from a blood vessel, and for restoring adequate blood flow to the blood vessel. One common procedure is known as percutaneous transluminal angioplasty (PTA), in which a catheter that is provided with a dilatation balloon at its distal end is positioned in a blood vessel at the site of the stenosis. The balloon is then expanded to dilate the blood vessel in order to restore adequate blood flow to regions beyond the stenosis.
While often effective PTA suffers from certain limitations. For example, PTA can be effective only if the occluding material in the blood vessel has a sufficiently large opening to allow the balloon to be positioned inside the occluding material. Where the blood vessel is almost completely occluded, it is difficult if not impossible to position the balloon of a catheter inside the occluding material. In addition, PTA suffers from multiple intra-procedural and post-procedural problems: abrupt closure, elastic recoil, and restenosis. Abrupt closure is the rapid reocclusion of the blood vessel within hours of the initial treatment. Abrupt closure can result from rapid thrombus formation which occurs in response to injury of the vessel wall from the PTA procedure. Elastic recoil is the elastic recovery of the dilated vessel approaching its pre-procedural diameter. Restenosis is the re-narrowing of the blood vessel over the weeks or months following an initial apparently successful PTA procedure. Restenosis occurs in up to 50% of all PTA patients and results from smooth muscle cell proliferation and migration and remodeling.
To overcome these limitations, a variety of catheters and techniques have been proposed which employ a removal mechanism to separate and remove occluding material from the luminal wall of the blood vessel. For example, rotational atherectomy devices (e.g., the Transluminal Extraction Catheter made by Interventional Technologies of San Diego, Calif., and the Rotablator, made by Boston Scientific of Bellevue, Wash.) rely on a rotating removal mechanism that can be advanced axially through vessels that are almost entirely occluded. The mechanisms, however, are available only in predetermined sizes, such as predetermined outer diameters. As a result, these removal mechanisms are most effective when used in blood vessels having a lumen size that approximate the size of the removal mechanism. These removal mechanisms are difficult to use in smaller-diameter blood vessels, and are less effective in removing occluding material from blood vessels having lumen sizes larger than the size of the removal mechanisms. This is a significant limitation since the lumen size, the size and extent of the occluding material, and the location of the occluding material, will vary widely for different patients.
To address this problem, many currently-available devices (e.g., the Simpson Atherocath.RTM. atherectomy catheter made by Guidant Corporation of Santa Clara, Calif., and the Redha-Cut, made by Sherine Med of Utzenstorf, Switzerland) are provided with eccentrically displaceable or radially expandable removal mechanisms respectively. These removal mechanisms are introduced into the blood vessel in a collapsed or compressed state inside a sheath or delivery catheter, and are then radially expanded or eccentrically displaced (e.g., displaced eccentrically by a balloon) at the site of the occluding material to separate and remove the occluding material. Unfortunately, the buildup and formation of the occluding material is rarely consistent since more occluding material may have formed at one location than at another location in the stenosed or occluded region of the blood vessel. Since the radially expandable removal mechanisms are typically expanded by the same radial distance throughout, the removal mechanism will remove material at a generally equal rate in all radial directions and may not be effective in removing all of the occluding material at locations where there has been a greater build-up of occluding material. If the procedure is continued until all material is removed, the removal mechanism may damage the vessel wall at locations where there was less build-up of occluding material.
In contrast, the eccentrically displaceable removal mechanisms can remove asymmetric build-ups of occluding material, but rely on the ability of the physician to orient the cutting window towards the occluding material, which may be at varying discrete locations along the length of the stenosis. Moreover, because they remove material in an asymmetric manner, if proper care is not exercised in their expansion, orientation and use, directional atherectomy devices can injure the luminal wall of the blood vessel (e.g. shaped wire multi-burr Rotational Ablation Device, U.S. Pat. No. 5,584,843, and the Abrasive Drive Shaft Device for Directional Rotation Atherectomy, U.S. Pat. No. 5,360,432). Thus, neither radially expandable nor eccentrically displaceable removal mechanisms are sufficiently capable of effectively adapting to the specific nature of the lumen and the occluding material.
The problems associated with proper sizing of the device, the nature and size of the occluding material, and the location of the occluding material are further magnified when treating stented regions of blood vessels that have restenosed. To address the problems of abrupt closure, elastic recoil, and restenosis described above, PTA procedures have been followed by implanting vascular stents inside the blood vessel at the treatment site. These stents are thin-walled scaffolds which are expanded at the treatment site to act as a mechanical support for the luminal wall of the blood vessel, thereby inhibiting elastic recoil. Although the stent diminishes the contribution of remodeling to vessel narrowing, restenosis still occurs frequently at the stented regions of blood vessels. This is because most stents comprise an open lattice, and cell proliferation (often referred to as hyperplasia) can occur in the interstices between the support elements of the lattice. As a result, instead of forming a barrier to hyperplasia and restenosis, the stent can become embedded within an accumulated mass of thrombus and tissue, and the treatment site becomes stenosed again. Treatment of an occluded stent faces all the difficulties discussed above with respect to treatment of initial occlusions and is further complicated by the need to avoid damaging the stent during the removal of the hyperplasia occluding material.
Thus, there remains a need for improved methods and apparatus for treating and removing occluding material from a blood vessel. In particular, it would be desirable to provide apparatus and methods which can remove material from vessels which are almost fully occluded, which can treat vessels having a range of sizes, and which can conform to a particular vessel size and lumenal shape during the course of a procedure. In addition, the apparatus and methods of the present invention should be effective for use in removing occluding material that engulfs an implanted stent. Desirably, the apparatus and methods of the present invention will be easy to implement, present acceptable risks to the patient, and be readily performed by physicians who are familiar with balloon angioplasty and other conventional intravascular treatments. At least some of these objectives will be met by the embodiments of the present invention described below.